History, principle, types, instrumentation, comparison with atomic emission spectroscopy, interference, advantages and disadvantages of different types of atomization techniques.
2. Introduction
• It is very reliable and simple to use
• It can analyze over 62 elements
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Atomic absorption spectroscopy is a
quantitative method of analysis of any kind of
sample ; that is applicable to many metals
3. HISTORY
• The technique was introduced in 1955 by Alan Walsh in
Australia ( 1916 – 1998 ).
• The first commercial atomic absorption spectrometer was
introduced in 1959.
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The application of atomic
absorption spectra to chemical
analysis
4. Principle
• The analyzed sample must contain the reduced metal in the
atomic vaporized state. Commonly, this is done by using the
heat of a flame to break the chemical bonds and form free,
unexcited atoms.
• The sample, in solution, is aspirated as a spray into a
chamber, where it is mixed with air and fuel. This mixture
passes through baffles, here large drops fall and are drained
off. Only fine droplets reach the flame.
• Light from the hollow-cathode lamp passes through the
sample of ground-state atoms in the flame. The amount of
light absorbed is proportional to the concentration.
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5. • When a ground-state atom absorbs light energy, an excited
atom is produced. The excited atom then returns to the
ground state, emitting light of the same energy as it
absorbed
• The flame sample thus contains a dynamic population of
ground-state and excited atoms, both absorbing and
emitting radiant energy. The emitted energy from the flame
will go in all directions, and it will be a steady emission
• Because the purpose of the instrument is to measure the
amount of light absorbed, the light detector must be able to
distinguish between the light beam emitted by the hollow
cathode lamp and that emitted by excited atoms in the
flame.
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6. • To do this, the hollow-cathode light beam is modulated by
inserting a mechanical rotating chopper between the light
and the flame or by pulsing the electric supply to the lamp
• Because the light beam being absorbed enters the sample in
pulses, the transmitted light also will be in pulses. There will
be less light in the transmitted pulses because part of it will
be absorbed. There are, therefore, two light signals from the
flame
• an alternating signal from the hollow-cathode lamp and
• a direct signal from the flame emission.
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14. LIGHT SOURCE
• Hollow Cathode Lamp are the most common radiation
source in AAS.
• It contains a tungsten anode and a hollow cylindrical
cathode made of the element to be determined.
• These are sealed in a glass tube filled with an inert gas (neon
or argon ) .
• Each element has its own unique lamp which must be used
for that analysis .
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15. • When voltage is applied, the filler gas is ionized. Ions
attracted to the cathode collide with the metal, knock atoms
off, and cause the metal atoms to be excited.
• When they return to the ground state, light energy is
emitted that is characteristic of the metal in the cathode.
• Generally, a separate lamp is required for each metal (e.g., a
copper hollow-cathode lamp is used to measure Cu)
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19. Sample Atomization Technique
Flame
Atomization
Electro thermal
Atomization
Hydride
Atomization
Cold-Vapor
Atomization
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Atomization is separation of particles into individual
molecules and breaking molecules into atoms. This
is done by exposing the analyte to high
temperatures in a flame or graphite furnace .
Atomization
20. Flame Atomization
• Nebulizer suck up liquid samples at controlled rate.
• Create a fine aerosol spray for introduction into flame.
• Mix the aerosol and fuel and oxidant thoroughly
for introduction into flame.
• An aerosol is a colloid of fine solid particles or liquid
droplets, in air or another gas.
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22. Disadvantages of Flame Atomization
• Only 5-15% of the nebulized sample reaches the flame.
• A minimum sample volume of 0.5-1.0 ml is needed to give
a reliable reading.
• Samples which are viscous require dilution with a solvent.
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23. Electro Thermal Atomization
• Uses a graphite coated furnace to vaporize the sample.
• ln GFAAS sample, samples are deposited in a small
graphite coated tube which can then be heated to vaporize
and atomize the analyte.
• The graphite tubes are heated using a high current power
supply.
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25. Advantages of Graphite Furnace
Technique
• Small sample size
• Very little or no sample preparation is needed
• Sensitivity is enhanced
• Direct analysis of solid samples
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26. Disadvantages of Graphite Furnace
Technique
• Analyte may be lost at the ashing stage
• The sample may not be completely atomized
• The precision is poor than flame method
• Analytical range is relatively low
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27. Hydride generation AAS
• A type of AAS where metal samples are vaporized by converting them
into volatile hydride.
•Hydride
•Any class of chemical compounds in which
hydrogen is combined with another element.
•Three types
•Ionic
•Metallic
•Covalent
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28. 28
Hydride generation
system
1.Pump
2.reactor
3,separator
1. Suck up
(aspirate)
liquid sample
at a
controlled
rate
4. Flow that
gaseous
hydride into
the optical
cell
3. Create a
volatile
hydride of the
analyte
metalloid from
that reaction
2. Mix liquid
sample with
sodium
borohydride
and HCl
29. MONOCHROMATOR
• This is a very important part in an AA spectrometer. It is used
to separate out all of the thousands of lines.
• A monochromator is used to select the specific wavelength
of light which is absorbed by the sample, and to exclude
other wavelengths.
• The selection of the specific light allows the determination
of the selected element in the presence of others.
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30. DIFFRACTION GRATING
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the process by which a beam of light or other system of waves is spread out
as a result of passing through a narrow aperture or across an edge, typically
accompanied by interference between the wave forms produced
31. DETECTOR
• The light selected by the monochromator is directed onto a
detector that is typically a photomultiplier tube , whose
function is to convert the light signal into an electrical signal
proportional to the light intensity.
• The processing of electrical signal is fulfilled by a signal
amplifier . The signal could be displayed for readout , or
further fed into a data station for printout by the requested
format.
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33. Calibration Curve
• A calibration curve is used to determine the unknown
concentration of an element in a solution.
• The instrument is calibrated using several solutions of known
concentrations.
• The absorbance of each known solution is measured and then a
calibration curve of concentration vs absorbance is plotted.
• The sample solution is fed into the instrument, and the
absorbance of the element in this solution is measured.
• The unknown concentration of the element is then calculated
from the calibration curve
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34. Applications
1) Presence of metals as an impurity or in alloys could be done
easily
2) Level of metals could be detected in tissue samples like
Aluminum in blood and Copper in brain tissues
3) Due to wear and tear there are different sorts of metals which
are given in the lubrication oils which could be determined for
the analysis of conditions of machines
4) Determination of elements in the agricultural and food
products
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36. COMPARISON
ATOMIC EMISSION
SPECTROSCOPY
• Examines the wavelengths of
photons emitted by atoms or
molecules during their transition
from an excited state to a lower
energy state.
• Each element emits a
characteristic set of discrete
wavelengths.
• By observing these wavelengths
the elemental composition of
the sample can be determined.
ATOMIC ABSORPTION
SPECTROSCPY
• Measures the loss of
electromagnetic energy after it
illuminates the sample under
study.
• The energy in certain amount
is absorbed during transition
to the higher level.
• The amount of energy
absorbed gives estimate of the
concentration of the analyte in
the sample.
37. “Increase or decrease in the size of the signal obtained from the
analyte as a result of the presence of some other known or unknown
component in the sample”
Types of interference:
Spectral interference
Chemical interference
38. Spectral interference
This may be caused by direct overlap of the analytical line with the
absorption line of the matrix element.
HOW TO OVERCOME ?
By choosing an alternate analytical wavelength
By removing the interfering element from the sample.
41. Chemical interference
Formation of compound
of low volatility
Decrease in calcium
absorbance is observed
with increasing
concentration of sulfate
or phosphate
42. By increasing flame temperature
Use of releasing agents (La 3+ )
Cations react with the interferent releasing the analyte
Use of protective agents:
They form stable but volatile compounds with analyte.
